exp 10 buckboost converter

8/13/2019 Exp 10 BuckBoost Converter

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EEK 471 LAB 10

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Universiti Sains MalaysiaElectrical Engineering DepartmentAdvanced Power Electronic Laboratory EEK471

The MOSFET Buck-Boost Chopper

OBJECTIVE

To learn the operation of a MOSFET buck-boost chopper.

INTRODUCTION

The buck-boost chopper

As discussed in the previous exercises of this manual, the buck chopper converts a dc voltageinto a lower dc voltage and the boost chopper converts a dc voltage into a higher dc voltage. Inthese choppers, the current always flows in the same direction, that is, from the input to theoutput of the chopper as shown in Figures 1(a) and (b).

In a buck-boost chopper, the current can flow in either direction. When the current flows in onedirection, the voltage is converted into a higher voltage, and therefore, the buck-boost chopperoperates as a boost chopper. On the other hand, when the current flows in the opposite direction,the voltage is converted into a lower voltage and the buck/boost chopper operates as a buckchopper. This is summarized in Figure 1(c).

Figure 1: Current flow in various types of choppers.


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A buck/boost chopper built with two MOSFETS and two diodes (free-wheeling diodes) is shown inFigure 2. This figure shows that the buck/boost chopper consists of a buck chopper and a boostchopper connected together. The buck chopper operates when the current flows from the high-voltage side to the low voltage side. In this case, the components of the boost chopper could beremoved without disturbing the operation of the circuit. Conversely, the boost chopper operateswhen the current flows from the low-voltage side to the high voltage side. In this case, thecomponents of the buck chopper could be removed without disturbing the operation of the circuit.

Figure 2 also shows the waveforms of the switching control signals applied to the MOSFETs.These signals are pulse trains whose duty cycles are complementary. For instance, when theduty cycle of one signal is 60%, the duty cycle of the other signal is 40%. Therefore, when oneMOSFET is switched on, the other MOSFET is switched off and vice versa.

The equation relating voltages V1and VOin the buck chopper and the equation relating voltagesV1 and VO in the boost chopper also apply for the buck-boost chopper. The duty cycle of theswitching control signal applied to the buck chopper MOSFET must be used when using the buckchopper equation. Similarly, the duty cycle of the switching control signal applied to the boostchopper MOSFET must be used when using the boost chopper equation.

Figure 2: A buck-boost chopper built with two MOSFETs and two diodes


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EQUIPMENTS

EMS 8821 Enclosure Power Supply

EMS 8840-0A PE Power Supply

EMS 8837-0A Power Mosfets Module

EMS 9029-00 Chopper/Inverter Control Unit

EMS 8412-05 Lab-Volt DC Voltmeter/Ammeter Fluke 87V True RMS Multimeter

EMS 8311 Variable Resistance X 2

EMS 8325 Smoothing Inductor

EMS 9056-15 and EMS 9056-05 Voltage/Current Isolator

24V AC Power Switch

Textronic Oscilloscope

Connection Leads

BNC-connector cable

WARNINGS

The voltages and currents that are used during this lab are larger and rated at 240VDCLine-to-Neutralwith current as high as 20 amps (or higher if circuits are improperly connected). Pleasetake the proper precautions and use your head before touching any circuitry. NEVERchange anycircuit connections while the power supply is turned on. Ask the demonstrator to check yourconnections before turning on the switches. And follow the rating of voltmeters and ammetersgiven to prevent equipments from damaged.

Experiment: Part IOperation of the MOSFET buck-boost chopper

1. In this part, you will use the circuit shown in Figure 1a to observe the operation of aMOSFET buck-boost chopper. In this circuit, the MOSFET buck-boost chopper isconnected between a voltage type source and a current-type source, which are both

current reversible. Connect the modules as shown in Figure 1a. Use Fluke 87V TrueMultimeter for reading I1dc.

2. Make the following settings on the Chopper/Inverter Control Unit

DC SOURCE 1Mid positionDC SOURCE 2MaximumMODE CHOP. PWM

3. Make sure that the main power switch of the Power Supply is set to the 0 (OFF) position.Set the voltage control knob to 0.

4. Set the 24V ac power switch to the 1 (ON) position. The POWER ON LED on the PowerMOSFETS module should light up to indicate that the module is correctly powered.


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Figure 1a: Circuit of the MOSFET buck-boost chopper

Figure 1b: L1= 0.8H

Table 1a: Parameters setting

E1(V) I1(A) I2(mA) L1(H) R1() RL()

300 mA/ A dc 300 0.8 686 1600

5. Set the main power switch to 1 (ON) and slowly set the voltage control knob to 20(%).This supplies dc power to the MOSFET buck-boost chopper. On the oscilloscope, makethe appropriate settings to position the traces of channels 1 and 2 in the upper and lowerhalves of the screen, respectively. The trace of channel 1 and 2 represent the switchingcontrol signal applied to MOSFET Q1andMOSFET Q4respectively.

6. On the Chopper/Inverter Control Units, set the DC SOURCE 2 control knob so that theperiod of the switching control signal to approximately 5 ms. Consequently, the operatingfrequency of the MOSFET buck-boost chopper is approximately 200 Hz.


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7. On the Chopper/Inverter Control Unit, slowly set the DC SOURCE 1 control knob so thatthe duty cycle of the switching control signal is equal to 50%. Sketch the waveforms ofthe switching control signals applied to MOSFETs Q1 and Q4 displayed on theoscilloscope screen in Figure 1c. From the waveforms sketched in Figure 1c, describethe switching sequence of the MOSFETs.

Figure 1c: Waveforms of the switching control signals in theMOSFET buck-boost chopper.

8. Measure and note the currents (l1dc and l2 dc) which the two ammeters indicate. Thecurrents l1dc and l2dc are the currents which flow on either side of the MOSFET buck-boost chopper.

II= ________ A dcI2= ________ mA dc

According to the polarity of the currents l1dc and l2dc, in which direction does the powerflow?

Measure and note the dc voltage indicated by the meter on the Power Supply. This is thevoltage at the terminals of dc power supply (E2dc). Measure and note the dc voltage at

load (E1dc) as well.

E1= ________ V dcE2= ________ V dc

Explain why the voltage at the terminals of the variable-voltage dc power supply (E2dc) isapproximately twice that produced by the load (E1dc).

9. On the Chopper/Inverter Control Unit, slowly set the DC SOURCE 1 control knob in bothdirections so that the duty cycle of the switching control signals varies between 30 and70%. Describe the switching control signals and the relationship between the duty cyclesof the two switching control signals.

10. On the Chopper/Inverter Control Unit, set the DC SOURCE 1 control knob so that theduty cycle of the switching control signals applied to MOSFETs Q1 is equal to 30%.Measure and note the dc voltage and current on either side of the MOSFET buck-boostchopper.

E1= ________ V dcE2= ________ V dcII= ________ A dcI2= ________ mA dc


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11. On the Chopper/Inverter Control Unit, set the DC SOURCE 1 control knob so that theduty cycle of the switching control signals applied to MOSFETs Q1 is equal to 60%.Measure and note the dc voltage and current on either side of the MOSFET buck-boostchopper.

E1= ________ V dcE

2= ________ V dc

II= ________ A dcI2= ________ mA dc

12. From the voltages measured in this step and the previous one, describe how the dcvoltage on either side of the buck-boost chopper varies when the duty cycles of theswitching control signals vary.

From the currents measured in this step and the previous one, describe how the dccurrent flowing in the buck-boost chopper varies when the duty cycles of the switchingcontrol signals vary.

13. On the Power Supply, set the voltage control knob to the 0 position then set the main

power switch and the 24 V ac power switch to the 0 position. Set the rocker switch on theEnclosure/ Power Supply to the 0 position. Remove all leads, cables and probes.

REVIEW QUESTIONS

1. Briefly describe the operation of the MOSFET buck-boost chopper.

2. The input supply of a buck-boost chopper is 40V. Knowing that the duty cycles of theswitching control signals applied to Q1and Q4are equal to 25 and 75%, respectively. Theresistive load RLis replaced by a 12-Vdc reversible dc power supply.

a) Is the 12-Vdc power supply supplying or receiving power?

b) Under the conditions stated in review question 2, would it be possible to removeMOSFET Q1 from the circuit without disturbing the operation of the MOSFET buck-boost chopper? Explain.

c) To which value cycles of the switching control signals of MOSFETS Q1and Q4be setso that no current flows in the buck-boost chopper.

3. What is the main feature which distinguishes the MOSFET buck-boost chopper from theMOSFET buck chopper and the MOSFET boost chopper?

exp 10 buckboost converter

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